1. Field of the Invention
The present invention relates to ethylene backbone copolymers, distillate oil compositions containing the copolymers, and an improved method of preparing these copolymers for use as pour depressants and flow improvers for distillate oil, which method involves free radical polymerization using an ester peroxide. These copolymers comprise about 3 to 40 molar proportions of ethylene per one molar proportion of an unsaturated monomer other than ethylene and have number average molecular weights in the range of about 1,000 to 50,000.
2. Description of the Prior Art
The commercially most important ethylene-containing pour depressant and flow improvers for distillate oil are copolymers of ethylene and ethylenically unsaturated ester monomers, such as the copolymers of ethylene and vinyl alcohol esters, particularly vinyl acetate, which copolymers are well known in the prior art. For example, U.S. Pat. No. 3,048,479 teaches copolymers of ethylene and C.sub.3 to C.sub.5 vinyl esters, e.g., vinyl acetate, having molecular weights of about 1,000 to 3,000 according to K. Rast's method of determining mol. wt. (Ber. 55, 1051, 3727, (1922)), as pour depressants for fuels, specifically heating oils, diesel and jet fuels. The copolymers of the examples of said patent were prepared by free radical catalysis, using ditertiary butyl peroxide as the catalyst, (although the patent teaches any peroxide catalyst), at temperatures of 280.degree. to 340.degree. F., in a solvent. U.S. Pat. No. 3,131,168 teaches a free radical process for making ethylene-vinyl acetate copolymers as pour depressants for middle distillate using temperatures up to 440.degree. F., a solvent such as toluene or hexane, any peroxy compound as catalyst, but preferably ditertiary butyl peroxide, and adding additional ethylene to the reaction during the polymerization. U.S. Pat. No. 3,093,623 teaches still another method for making these ethylene-vinyl acetate pour depressants for middle distillates by continuously adding vinyl acetate and ethylene during the course of the reaction. U.S. Pat. No. 3,250,714 teaches ethylene-vinyl acetate copolymers having molecular weights of 3500 to 7000 as V.I. improvers for lubricating oils.
More recently, British Pat. Nos. 1,263,151 and 1,263,152 teach an improvement over the aforesaid U.S. patents by using a polymerization temperature below about 130.degree. C. and acyl peroxide as the initiator, as opposed to the alkyl peroxide, tert. butyl peroxide, and higher temperatures used by the aforesaid U.S. patents. By the technique of said British Patents, it was found that the amount of ethylene branching was considerably reduced and copolymers produced by this method were generally superior pour point depressant and flow improvers to prior art copolymers prepared at higher temperatures with alkyl peroxides. Specifically, copolymers prepared with alkyl peroxides and high temperatures, while very effective in treating distillate fuel oil to lower the pour point, frequently result in wax crystals having large particle sizes ranging from one millimeter up to an inch in their largest dimension, depending upon the exact nature of the distillate oil, e.g., crude source, narrowness of the boiling range, etc. While the treated distillate oil containing these large wax crystals exhibits a pour point significantly under the original pour point of the untreated oil, in many cases, the large wax crystals will tend to plug filter equipment and lines normally used on delivery trucks and fuel oil storage systems when the oil is cooled below its cloud point, even though above its pour point. Thus, as the oil containing the pour point depressant is cooled, the cloud point (the point at which the oil becomes cloudy due to crystallization of wax) will generally be reached at a temperature significantly above the pour point (the point at which the oil can no longer conveniently be poured). As a result, oils below their cloud point and above their pour point will be pourable, but at the same time the wax crystals that have formed, if too large, can result in plugging the aforesaid filter equipment. Copolymerizing ethylene and vinyl acetate at a low temperature with the acyl peroxides of said British Patents was able to give the good pour point reduction and in addition form smaller wax crystals during cooling of the treated oil.